1. Technical Field of the Invention
The present invention relates generally to perforation guns that are used in the oil and gas industry to explosively perforate well casing and underground hydrocarbon bearing formations, and more particularly to an improved method and an improved apparatus for explosively perforating a well casing and its surrounding underground hydrocarbon bearing formation while limiting the amount of explosion debris in the well bore and hydrocarbon bearing formation following perforation.
2. Description of the Related Art
During the completion of an oil and/or gas well, it is common to perforate the hydrocarbon containing formation with explosive charges to allow inflow of hydrocarbons to the well bore. These charges are loaded in a perforation gun and are typically shaped charges that produce an explosive formed penetrating jet in a chosen direction.
FIG. 1 illustrates a perforation gun consisting of a cylindrical carrier 14 hanging from a cable 20. At the well site, the explosive charges 16 are placed into the charge carrier 14, and the charge carrier 14 is then lowered into oil and gas well casing to the depth of the hydrocarbon bearing formation 12. The exploding charges 16 fire outward from the charge carrier 14 and the force from each charge punctures holes 24 in the wall 18 of the casing and the hydrocarbon bearing formation 12, which allows oil, gas, water and/or minerals to flow into the casing from the hydrocarbon bearing formation 12.
While perforation guns do increase fluid production from hydrocarbon bearing formations, the effectiveness of traditional perforation guns is limited by the fact that the firing of a perforation gun can leave behind “debris” inside the casing and the hydrocarbon bearing formation 12. This debris can cause significant operational difficulties for the well operator and has to be cleaned out of the well at a significant cost. FIG. 2 shows a traditional hollow carrier perforation gun 14B, positioned adjacent to a hydrocarbon bearing formation 12 as shown in FIG. 1, after it has been fired and the explosive charge receiving areas 16B have been damaged. The debris 22 left behind is essentially blast shrapnel, which are pieces of the charge carrier 14B, the explosive charges, and the explosive charge receiving areas 16B that obstruct the production of oil and gas from the well.
Prior art has proposed an apparatus used to trap this debris before it enters the well casing and hydrocarbon bearing formation, which is disclosed in Rospek et. al. PCT Application WO 2005/033472. FIG. 3 is a depiction of the perforation gun described in Rospek '472. It is composed of an outer gun barrel 210 with a coaxial interior hollow charge carrier 212. The explosive charges 214 are inside the charge carrier 212. FIG. 4 shows the perforation gun of FIG. 3 as it is being fired. When the explosive charges 214B contained inside the charge carrier 212 are detonated, the explosions 226 create holes 224 in both the interior charge carrier 228 and the outer gun barrel 230. The perforation gun then theoretically traps the debris 22 from the detonation within the charge carrier 212 by moving the entire charge carrier 212 (including the charge-housing portion 232 of the gun that originally housed the explosive charges) along the axis it shares with the outer gun barrel 210 until the holes 224 created by the charges 228 and 230 are no longer aligned. The Rospek '472 application discloses that the movement of the interior charge carrier 212 can theoretically be actuated using explosives, a strained spring, or the force from the explosive charges 214B. Such actuating force must be great enough to break the shear pin 216, 216B and move the charge carrier 212 a distance 222 (which distance must be large enough to allow movement sufficient to seal the holes created by the charges 228 and 230) until the bottom surface 218 of the charge carrier 212 impacts the endplate 220. The Rospek '472 application also teaches that the walls of both the outer gun barrel 210 and the charge carrier 212 are solid before the perforation charges 214 are detonated.
A need exists for an improved and more comprehensive and more efficient design for a debris trapping perforation gun. A further need exists for an improved, more comprehensive and more efficient method of operation of the debris trapping perforation gun.